Pivotal barrel and receiver assembly having a removably mounted breech block

ABSTRACT

A double-barreled firearm of the over-under barrel configuration utilizing a break-open type action wherein the barrels are hinged on trunnions fixed to the receiver. The receiver is formed with a top and two parallel depending sides having a machined channel therein for slidably receiving a removable breech block. Openings are provided in the breech block for passage of barrel locks extending past the breech face and engaging detents in the breech end of the barrel housing. A removable trigger group is releasably secured to the bottom of the receiver and includes a trigger, a sear actuator, a pair of sears, and a pair of hammers; a selector/safe carriage is also provided within the trigger group and is slidably positionable to select either of the sears for actuation while automatically locking the non-selected sear. The selector/safe carriage may also be positioned to lock the trigger to prevent firing of either barrel. The positioning of the selector/safe carriage is achieved through the utilization of a pivoted cam actuated by a manual safe/selector switch located on top of the receiver. A release lever is also positioned on the top of the receiver for withdrawing the barrel locks from the detents in the barrel housing; an extension having a camming surface is secured to the barrel locks for engaging the cam following surface on the selector/safe mechanism to automatically position the latter in the same position at any time the barrel locks are disengaged from the barrel housing. The distance from the trunnions to the machined channels in the receiver is accurately controlled to thus permit interchangeability of different size barrels with a given receiver.

United States Patent [1 1 Waddell Oct. 23, 1973 PIVOTAL BARREL AND RECEIVER ASSEMBLY HAVING A REMOVABLY MOUNTED BREECH BLOCK [75] lnventor: Richard G. Waddell, Phoenix, Ariz.

[73] Assignee: The Innovation Resources Corporation, Phoenix, Ariz.

[22] Filed: Sept. 7, 1971 [21] Appl. No.: 178,268

Primary ExaminerBanjamin A. Borchelt Assistant ExaminerC. T. Jordan Attorney-William C. Cahill et a1.

[57] ABSTRACT A double-barreled firearm of the over-under barrel configuration utilizing a break-open type action wherein the barrels are hinged on trunnions fixed to the receiver. The receiver is formed with a top and two parallel depending sides having a machined channel therein for slidably receiving a removable breech block. Openings are provided in the breech block for passage of barrel locks extending past the breech face and engaging detents in the breech end of the barrel housing. A removable trigger group is releasably secured to the bottom of the receiver and includes a trigger, a sear actuator, a pair of sears, and a pair of hammers; a selector/safe carriage is also provided within the trigger group and is slidably positionable to select either of the sears for actuation while automatically locking the non-selected sear. The selector/safe carriage may also be positioned to lock the trigger to prevent firing of either barrel. The positioning of the selector/safe carriage is achieved through the utilization of a pivoted cam actuated by a manual safe/selector switch located on top of the receiver. A release lever is also positioned on the top of the receiver for withdrawing the barrel looks from the detents in the barrel housing; an extension having a camming surface is secured to the barrel locks for engaging the cam following surface on the selector/safe mechanism to automatically position the latter in the same position at any time the barrel locks are disengaged from the barrel housing. The distance from the trunnions to the machined channels in the receiver is accurately controlled to thus permit interchangeability of different size barrels with a given receiver.

8 Claims, 12 Drawing Figures Patented Oct. 23, 1973 6 Sheets-Shoot l a E m m TM" E N W5 ND 7 IPA M 4 m R Patented Oct. 23, 1973 3,766,677

6 Shasta-Shut 2 I NVENTOR RICH/12D 6. WADDELL Patented 0a. 23, 1973 3,766,677

6 Sheets-Shut 5 INVENTOR RI H/4RD 6. WADDEZL Patented Oct. 23, 1973 3,766,677

6 Sheets-Shoot 5 INVENTOR RICHARD 6. WA 0051.].

M/w/m W AITJRNEYS Patented Oct. 23, 1973 6 Shaats-Shoot G E/C/IARD 6. WADDELL Wm, M w

AITOENEY PIVOTAL BARREL AND RECEIVER ASSEMBLY HAVING A REMOVABLY MOUNTED BREECII BLOCK BACKGROUND OF THE INVENTION The present invention pertains to firearms, and more particularly, to a break-open type firearm utilizing interchangeable barrel sets as well as multiple barrel firearms having a barrel firing order selection means.

Double-barreled firearms have been long known and used; the desirability of such an arm, particularly the over-under configuration, is evidenced by the continued demand for such arms for an extensive number of years. However, such arms are usually expensive as a result of the relatively large amount of hand-fitting and manual labor involved in their manufacture. The most common double-barreled configuration in the overunder variety is a shotgun cartridge of the same gauge in each barrel wherein each incorporates a different choke. The utilization of such firearm configuration for high-powered rifle cartridges is more limited since the pressures involved with such cartridges are more demanding of the strength of the action and the tolerances utilized in the manufacture of the arm.

In firearms of this type, a significant contribution to the required labor in the manufacture thereof results from the tolerance requirements of the head space. In break-open type actions, particularly the over-under variety, this head space is controlled by the distance between the trunnion pins upon which the barrel housing is pivoted and the breech face. Since the receiver is formed with side walls extending forwardly from the breech face to support the trunnions, machining the breech face to close tolerance relative to the positioning of the trunnions becomes a very difficult task requiring the skill of a technician to hand-fit the barrel housing to the receiver. It is well known that such double guns may be obtained with interchangeable barrel groups; however, such interchangeable or substitute barrels must each be hand-fit to the individual receiver and cannot be provided as an off-the-shelf accessory.

The removal of the operating mechanism of a firearm of the type described is of critical importance in the maintenance of the firearm. The complexity of this mechanism positioned behind the machined breech face often necessitates the attention of a skilled gunsmith for removal and inspection or repair. It is also desirable to incorporate as a function of this mechanism the ability to select a barrel for firing and to provide a means for positively locking the sears when the firearm is placed on safe through a conveniently positioned safety mechanism.

OBJECTS OF THE INVENTION It is therefore an object of the present invention to provide a firearm susceptible of using interchangeable barrels without the necessity of fitting each barrel housing to a respective receiver.

It is still another object of the present invention to provide a firearm of the break-open variety wherein the distance between the barrel hinging trunnions and the breech face may be consistently controlled within desired limitations to provide dimensional uniformity among successively manufactured receivers.

It is still another object of the present invention to provide a firearm incorporating a breech block having a breech face that may accurately be machined prior to insertion in the receiver.

It is still a further object of the present invention to provide a firearm incorporating a breech block that may readily be removed for inspection and/or repair.

It is a further object of the present invention to provide a multiple barrel firearm of the break-open variety incorporating a barrel selector mechanism that automatically provides a positive safety when the action is broken.

It is another object of the present invention to provide a multiple barrel firearm incorporating a selector mechanism that automatically places the non-selected barrel into a positive safe condition.

It is still another object of the present invention to provide a firearm wherein the trigger-sear-hammer mechanism as well as the selector/safe mechanism is readily removable from the receiver without the utilization of tools.

These and other objects and advantages of the present invention will become apparent to those skilled in the art as the description thereof proceeds.

SUMMARY OF THE CHOSEN EMBODIMENT Briefly, in accordance with the embodiment chosen for illustration, an over-under firearm of the breakopen type is provided with a solid receiver having a top and depending side walls with the side walls extending forwardly of the top to form a barrel-receiving channel. Trunnions are fitted in the forward portion of the side walls and a pair of opposed machined channels are formed in the interior of the side walls beneath the receiver top. The channels may be formed such as by broaching and slidably receive a pre-machined breech block having firing pins mounted therein. The breech block is secured in the receiver through the utilization of a pin passing through the lower portion thereof. A trigger group including a frame is secured through an opening in the bottom of the receiver through the utilization of a latch. The trigger group includes a pair of hammers pivotally secured to the frame and a corresponding pair of sears. The sears incorporate exten sions which are contacted by a sear actuator for disengaging the sear with the corresponding hammer. A selector/safe mechanism including a selector/safe carriage is mounted in the frame on a pair of transverse pins which permit the carriage to be positioned laterally of the receiver. The selector/safe carriage and the sear actuator move in unison with the selector carriage engaging either sear extensions to place the corresponding sear, hammer, and barrel in the safe position.

The removable breech block includes a pair of L- shaped shoulders extending perpendicular to the breech face and which form rectangular openings extending through the breech face and breech block when the breech block is inserted in the receiver, the rectangular openings or passages being bordered on the bottom and one side by the breech block, on one side by the receiver side plate, and on the top by the receiver top. A pair of barrel locks slidably extend through these passages and engage detents provided in the breech end of the barrel housing for positively locking the barrels into a firing position. The action may be broken by actuation of a release lever operating in either direction which operates a cam to withdraw the barrel locks. A barrel lock extension cams the selector/safe carriage to its central or safe position, thereby placing both barrels in a positive safe position automatically upon breaking the action. The selector/safe carriage may be placed in a desired barrel firing position through the utilization of a selector button extending through the top of the receiver.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention may more readily be described by reference to the accompanying drawings, in which:

FIG. 1 is a side elevational view of a firearm incorporating the teachings of the present invention, showing a shotgun-over-shotgun configuration.

FIG. 2 is a side elevational view of a barrel group of the rifle-over-rifle configuration that may be utilized interchangeably with the barrel group of FIG. 1.

FIG. 3 is a cross section of the barrel band utilized in the embodiment of FIG. 2.

FIG. 4 is an exploded perspective view of the receiver utilized in the present invention, showing the removable breech block and other mechanism.

FIG. 5 is an exploded perspective view of the removable breech block used in the present invention, showing the firing pins carried by the breech block.

FIG. 6 is a partly exploded and partly sectional view of a firearm constructed in accordance with the teachings of the present invention.

FIG. 7 is an exploded view of the release lever and selector/safe selection mechanism.

FIG. 8 is a side elevational view of the trigger group of the present invention.

FIG. 9 is an end view of the apparatus of FIG. 8, looking toward line 9 -9.

FIG. 10 is an exploded perspective view of the trigger group of FIGS. 8 and 9.

FIG. 11 is a side elevational view, partly in section and partly exploded, of a portion of the firearm of FIG. 1.

FIG. 12 is an elevational view of the breech end of the barrel housing of FIG. 11, viewing from line 12-12.

DETAILED DESCRIPTION OF THE DRAWINGS Referring now to FIG. 1, the firearm shown therein is of the over-under variety having both barrels l0 and 11 accepting shotgun type cartridges. The barrels are permanently attached to a barrel housing (not shown in FIG. 1) which is removably and pivotally secured to the receiver 12 by trunnion lugs such as that shown at 14. A forearm 15 is provided which, in turn, is attached to a forearm iron 16, the latter being secured to the barrels 10 and 11. A trigger guard 18 surrounds a conventional trigger 19, the two being part of a trigger group (not shown in FIG. 1) which is removably secured in the receiver 12 by a latch mechanism actuated by trigger group release 21.

A release lever 24 enables the action to be broken and a selector/safe lever 25 enables the operator to select a safe position or a position for firing either of the two barrels. A conventional stock 26 is secured to the receiver 12 in the well-known manner.

Referring to FIG. 2, an interchangeable barrel set is shown. The barrel set of FIG. 2 is of the rifle-over-rifle variety wherein each barrel accepts a rifle cartridge of any predetermined caliber; the calibers may be the same or different. The barrels 28 and 29 are each fitted to a barrel housing 30 and are tied to each other near the muzzle thereof by an adjustable barrel band 32.

When utilizing a rifle-over-rifle configuration, convergence of the trajectories of the projectiles emanating from the respective barrels becomes important. This convergence may be factory adjusted or, as shown in FIG. 2, it may be achieved through the utilization of the barrel band 32. The band may more readily be described by reference to FIG. 3, wherein it may be seen that an upper ring portion 34 is provided for surrounding the upper barrel and may be clamped to the barrel in any convenient fashion, such as by utilizing the already existing front sight ramp. A lower ring 36 encircles the lower barrel of the pair of barrels and is provided with two pairs of diametrically opposed set screws 37, 38, 39, and 40. By loosening and tightening appropriate pairs of the set screws 37 40, the lower barrel may be biased to cause any desired convergence of projectile trajectories of the two barrels.

Referring now to FIGS. 4 10, it may be seen that the receiver 12, in the embodiment chosen for illustration, is machined from a single piece of steel and incorporates a top 50 with side plates 52 and 54 extending downwardly therefrom. The side plates 52 and 54 also extend forwardly of the top 50 to form an open top barrel receiving channel 56. The forward extending portions of the side plates 52 and 54 are appropriately drilled to receive trunnion lugs 58 and 59; extractor earns 61 and 62 are secured adjacent the trunnion lugs. The inside surface of the side plates 52 and 54 are machined to provide a pair of oppositely disposed channels 64 and 65, said channels facing one another and extending generally vertically from the bottom of the receiver 12 to adjacent the top 50 of the receiver.

An insertable breech block is premachined and includes a breech face 71 having firing pin holes 73 and 74 therein. The breech block is slidably positioned in the channels 64 and 65 and is pinned in place through the utilization of pin 76 extending through the side plate 54, breech block 70, and side plate 52. The positioning of the channels 64 and 65 with respect to the center line 78 of the trunnions 58 and 59 may very accurately be controlled using present machining techniques. This positioning may be achieved with a significant degree of repeatability, thus producing receivers such as receiver 12 having a predetermined trunnionto-breech face dimension well within the tolerances to accept mass-produced barrel sets.

The breech block 70 also includes firing pins 80 and 81 as well as firing pin springs 82 and 83 respectively. The firing pins and their springs are held within the breech block 70 by retaining pins 85 and 86. The pin 76 extending through the side walls 52 and 54 of the receiver l2 and passing through the breech block 70 also acts as a pivot pin for a cocking arm 88 and secures cover plate 90 in place. The trigger group 100, in addition to the trigger guard 18 and trigger 19, includes a frame 101. The trigger 19 is pivotally secured to the frame 101 by a trigger pin 103; the actuating surface 104 of the trigger 19 engages a sear actuator 106 which is pivotally mounted to the frame 101 by pivot pin 107. A trigger spring 108 urges the trigger in a clockwise di rection as viewed in the figures. The sear actuator 106 is positioned between locking extensions 110 and 111 of the selector/safe carriage 1 12. The selector/safe carriage 112 is also mounted on pin 107 but is restrained from rotation or pivotal motion (such as the pivoting motion of the sear actuator 106) by a second pin 114 passing through slots 115 and 116 in the rear of the selector/safe carriage 112. The carriage 112 is thus free to slide transversely on the pins 107 and 114, together with the sear actuator 106, but only the sear actuator is free to pivot about the pin 107.

Sears and 121 are mounted for pivotal movement about sear pin 123. The sears each include sear extensions 124 and 125 respectively which extend away from the sear pin 123 and permit the respective sear to be pivoted about the sear pin by engaging the sear actuator 106. Sear springs 128 and 129 urge the respective sears into the hammer-engaging position; the sear springs are maintained in position by a spring retainer block 130. The retainer block 130 is secured to the frame 101 by sliding the block laterally through the openings 134 and 135 provided in the frame 101. The spring retainer block 130 is also provided with openings 138 and 139 for slidably receiving hammer guide rods 140 and 141 respectively. The hammer guide rods are centrally and co-axially positioned with respect to hammer springs such as spring 151 (only one hammer spring is shown).

The hammer guide rods 140 and 141 include hammer-engaging tips 153 and 154 which engage hammers 155 and 156 respectively. The hammers are pivotally secured to the frame 101 by hammer pin 160.

The frame 101 is secured in the bottom of the receiver 12 by engaging the slightly tapered rear edge 163 with a corresponding tapered surface 164 at the rear of the receiver 12 and pivoting the frame upwardly until the camming surface 166 of the trigger group release 21 engages the edge 167 of the cover plate 90. Continued rotation of the' frame 101 causes the compression of spring 169 until the trigger group release snaps into locking engagement with the cover plate 90 and with the inclined surfaces 170 and 171 abutting corresponding mating surface 174 in the receiver 12.

The safety/selector lever 25 extends through the top of the receiver and is slidable transversely of the receiver 12 in the direction indicated by the arrow (FIG. 7). This transverse motion is transmitted to a selector/safe block 181 through a pin 182. The block is secured to the bottom surface 183 of the receiver top 50 by a pivot pin 185 which results in a pivotal motion of the block 181 when the selector/safe lever 25 is positioned. A selector pin 187 is secured to the block 181 and extends downwardly therefrom to a position between the locking extensions 110 and 111 of the selector/safe carriage 112 and over the sear actuator 106. The transverse motion of the selector/safe lever 25 therefore results in pivotal motion of the selector/safe block 181 and actuator pin 187 which, in turn, results in transverse sliding motion of the selector/safe car riage 112 on the pins 107 and 114. Since the sear actuator 106 is also mounted between the locking extensions 110 and 111, the former slides transversely with the carriage 112. It may be noted that the locking extensions 110 and 111 include notches such as notch 200 which slide over the ends of the sear extensions 124 and 125. When selector/safe carriage 112 is positioned centrally (the safe position), the selector pin 187 is in an interfering position directly over the sear actuator 106 and prevents the actuator from being pivoted to fire either barrel. Sliding the selector/safe carriage 112 to the left or right moves the selector pin 187 out of interfering relation with the sear actuator 106. It may be noted that the top of the sear actuator 106 is notched at 106a and 106b on either side of a raised center portion 109. The pin 187 therefore prevents operation of the actuator 106 only when the pin 187 is directly over the center portion 109. If the pin 187 is left or right of the raised center portion, then the pin is aligned with one of the notches 106a or 106b and the actuator 106 may be operated. Simultaneously with this movement to a non-interfering position of the selector pin 187, the sear actuator 106 is positioned immediately above that sear extension 124 or 125 that is selected while the notch in locking extension 110 or 111 will engage the non-selected sear extension and lock that sear against pivotal movement. Pivoting of the trigger 19 therefore results in pivoting of the sear actuator 106, causing contact of the actuator with the end of the selector scar and pivoting of the sear from its engaged to its disengaged position; the non-selected sear remains locked against pivotal movement, thus preventing discharge of the non-selected barrel such as by doubling.

The selector/safe lever 25 is maintained in any of three selected positions through a spring-loaded ball 202 mounted on the selector/safe block 181, which ball engages one of three detents 203, 204, or 205 formed in the bottom surface 183 of the receiver top 50. A release lever 206 is positioned externally of the top 50 of the receiver 12 and extends therethrough and is connected to a locking cam 208. A pair of barrel locks 210 and 211, in the embodiment chosen for illustration, are formed of a rectangular cross section and include tapered faces 212 and 213 respectively. A cam following surface 215 is positioned relative to the cam 208 so that rotation of the lever 206 results in the engagement of the cam 208 with the surface 215 and causes the barrel locks 210 and 211 to move to the right as viewed in the figures. The barrel locks are urged into an extended position wherein they extend outwardly of the breech face 71 through the utilization of a barrel lock spring 218 compressed between a shoulder 220 provided in the bottom surface 183 of the top 50 of the receiver 12 and an opposing recess 221 positioned opposite the camming surface 215. A barrel locking extension 225 extends rearwardly from the barrel locks 210 and 21 1 and includes a camming surface 227. Rearward motion of the extension 225 results in the engagement between the camming surface 227 and a cam following surface 230 provided on the forward surface of the selector/- safe block 181. Therefore, rearward motion of the barrel lock extension 225 causes the selector/safe block 181 to assume its central or safe position, forcing the selector/safe carriage 112 to assume a like safe position. Thus, upon breaking the action of the firearm of the present invention, sears in the engaged position are automatically prevented from being released by locking the sear actuator.

The breech block 70 includes a pair of L-shaped shoulders 235 and 236 machined therein extending perpendicularly to the breech face 71. When the breech face 70 is inserted in the channels 64 and 65, the L-shaped shoulders 235 and 236 form rectangular openings passing through the breech block from the face to the rear thereof. These openings take a substantially rectangular form, each having one of the sides and the bottom thereof formed by the L-shaped shoulders, while the outside side and the top thereof are formed by one of the side plates 52 or 54 of the receiver 12 and the top formed by the top 50 of the receiver 12. This rectangular opening through the breech block permits the utilization of barrel locks 210 and 211 in the form shown. Such a rectangular opening through the breech would be nearly impossible to machine in prior art designs. The barrel locks 210 and 211 present slightly tapered surfaces 212 and 213 so that the locks are readily camrned back into the breech block as the action is closed until the barrel locks 210 and 211 snap" into engagement with corresponding detents 240 and 242 provided in the barrel housing 250 (FIG. 12).

Referring now to FIGS. 11 and 12, the barrels l and 11 are attached to barrel housing 250 to which a cocking lever 254 is slidably secured. The cocking lever engages the cocking arm 88 when the action is broken, resulting in pivotal movement of the cocking arm 88 to force the hammers 155 and 156 (not shown in FIGS. 11 and 12) back into their sear-engaging positions. In the embodiment chosen for illustration, a pair of extractors 260 and 261 are shown which are slidably positioned alongside the barrel housing 250. The extractors are urged to their extracted position by engagement between one of the extractor earns 61 or 62 (FIG. 4) and a cam contacting surface, such as camming arm 267. It will be obvious to those skilled in the art that an ejector mechanism may be coupled with the extraction mechanism above-described utilizing any of several wellknown ejection designs which may incorporate springs, such as spring 268 engaging extraction rod extension 270.

A forearm 278 incorporating a latch 278 for engaging hook 279 is provided and includes a fore iron 280 incorporating an appropriately curved surface 285 for following corresponding curved surface 287 (FIG. 4) formed in the forward extending side walls 52 and 54. The trunnion lugs 58 and 59 engage arcuate lug receivers such as receiver 290 formed in the forward portion of the barrel housing 250. The barrel housing, and thus the barrels, pivot about the trunnion lugs 58 and 59 which, positioned in the manner described relative to the insertable breech block 70, provides accurate positioning of the barrel housing breech face 292 relative to the breech face 71 without the previously necessary hand-fitting of the barrel housing 250 and connected barrels l0 and 11 to the receiver 12.

I claim:

1. In a firearm, the combination comprising: a receiver having a top and two side plates, said side plates extending downwardly from said top and forwardly thereof to form an open top barrel receiving channel; a pair of oppositely disposed trunnions, one in each forwardly extended side plate for receiving a barrel pivotally supported by said trunnions; a pair of oppositely disposed machined channels, one formed in each side plate, said machine channels opening toward each other and positioned beneath said top and at predetermined distances from said trunnions, said machined channels extending substantially transverse to the axis of a barrel positioned in said barrel receiving channel; a breech block extending between and removably positioned in said machined channels and having a breech face for abutting the breech end of the barrel, said breech block remaining stationary during functioning of the firearm; means removably securing said breech block in said machined channels; locking means for locking said barrel in a position with the breech end thereof opposite said breech face.

2. the combination set forth in claim 1, wherein said machined channels are rectangular in cross section and extend downwardly from beneath said top to receive and position said breech block from the bottom of said receiver.

3. The combination set forth in claim 1, including a firing pin mounted within said breech block and means for retaining said firing pin in said breech block.

4. The combination set forth in claim 1, wherein said breech block incorporates a pair of oppositely disposed L-shaped shoulders extending perpendicular to said breech face, each positioned adjacent a side and top of said breech block; said shoulders forming, with said receiver sides and top, two rectangular openings extending perpendicularly through said breech face and breech block, and wherein said locking means comprises a pair of locking arms slidably mounted in said rectangular openings and spring-biased to extend outwardly of said breech face for engaging the breech end of said barrel and holding said barrel in a closed position.

5. The combination set forth in claim 1, wherein said breech block incorporates at least one opening extending therethrough perpendicular to said breech face and wherein said locking means comprises a locking arm slidably mounted in said opening and spring-biased to extend outwardly of said breech face for engaging the breech end of said barrel and holding said barrel in a closed rectangular position.

6. The combination set forth in claim 2, including a firing pin mounted within said breech block and means for retaining said firing pin in said breech block.

7. The combination set forth in claim 2, wherein said breech block incorporates a pair of oppositely disposed L-shaped shoulders extending perpendicular to said breech face, each positioned adjacent a side and top of said breech block; said shoulders forming, with said receiver sides and top, two rectangular openings extending perpendicularly through said breech face and breech block, and wherein said locking means comprises a pair of locking arms slidably mounted in said rectangular openings and spring-biased to extend outwardly of said breech face for engaging the breech end of said barrel and holding said barrel in a closed rectangular position.

8. The combination set forth in claim 2, wherein said breech block incorporates at least one opening extending therethrough perpendicular to said breech face and wherein said locking means comprises a locking arm slidably mounted in said opening and spring-biased to extend outwardly of said breech face for engaging the breech end of said barrel and holding said barrel in a closed rectangular position. 

1. In a firearm, the combination comprising: a receiver having a top and two side plates, said side plates extending downwardly from said top and forwardly thereof to form an open top barrel receiving channel; a pair of oppositely disposed trunnions, one in each forwardly extended side plate for receiving a barrel pivotally supported by said trunnions; a pair of oppositely disposed machined channels, one formed in each side plate, said machine channels opening toward each other and positioned beneath said top and at predetermined distances from said trunnions, said machined channels extending substantially transverse to the axis of a barrel positioned in said barrel receiving channel; a breech block extending between and removably positioned in said machined channels and having a breech face for abutting the breech end of the barrel, said breech block remaining stationary during functioning of the firearm; means removably securing said breech block in said machined channels; locking means for locking said barrel in a position with the breech end thereof opposite said breech face.
 2. The combination set forth in claim 1, wherein said machined channels are rectangular in cross section and extend downwardly from beneath said top to receive and position said breech block from the bottom of said receiver.
 3. The combination set forth in claim 1, including a firing pin mounted within said breech block and means for retaining said firing pin in said breech block.
 4. The combination set forth in claim 1, wherein said breech block incorporates a pair of oppositely disposed L-shaped shoulders extending perpendicular to said breech face, each positioned adjacent a side and top of said breech block; said shoulders forming, with said receiver sides and top, two rectangular openings extending perpendicularly through said breech face and breech block, and wherein said locking means comprises a pair of locking arms slidably mounted in said rectangular openings and spring-biased to extend outwardly of said breech face for engaging the breech end of said barrel and holding said barrel in a closed position.
 5. The combination set forth in claim 1, wherein said breech block incorporates at least one opening extending therethrough perpendicular to said breech face and wherein said locking means comprises a locking arm slidably mounted in said opening and spring-biased to extend outwardly of said breech face for engaging the breech end of said barrel and holding said barrel in a closed rectangular position.
 6. The combination set forth in claim 2, including a firing pin mounted within said breech block and means for retaining said firing pin in said breech block.
 7. The combination set forth in claim 2, wherein said breech block incorporates a pair of oppositely disposed L-shaped shoUlders extending perpendicular to said breech face, each positioned adjacent a side and top of said breech block; said shoulders forming, with said receiver sides and top, two rectangular openings extending perpendicularly through said breech face and breech block, and wherein said locking means comprises a pair of locking arms slidably mounted in said rectangular openings and spring-biased to extend outwardly of said breech face for engaging the breech end of said barrel and holding said barrel in a closed rectangular position.
 8. The combination set forth in claim 2, wherein said breech block incorporates at least one opening extending therethrough perpendicular to said breech face and wherein said locking means comprises a locking arm slidably mounted in said opening and spring-biased to extend outwardly of said breech face for engaging the breech end of said barrel and holding said barrel in a closed rectangular position. 